Mobile satellite communications (SATCOM) is emerging as an increasingly important upcoming technology, and low-profile antennas figure to play a prominent role in mobile SATCOM. These low-profile systems and antennas are highly desired for aeronautical applications in order to minimize drag and reduce fuel consumption. Such antennas also enable lower profile protective radome enclosures, significantly lowering the overall operational costs of the antenna system.
Variable inclination continuous transverse stub (VICTS) antennas are extremely low profile phased array antennas with low loss and excellent gain. A compact, low-profile waveguide rotary joint is an important component in these antennas since the antennas operate by rotating individual platters with respect to one another to electromechanically steer a main beam to a target satellite. The input waveguide feeding structure for these antennas is normally located away from the rotational center and as such, a waveguide rotary joint structure which provides a continuous free-rotating microwave connection between the rotating feed structure and the fixed antenna mount is needed.
Existing waveguide rotary joints are not only relatively large, but also expensive. For ground mobile applications, low-profile antennas are highly desired not just for aesthetics but also to reduce drag and fuel consumption when vehicles are in motion. For aeronautical applications, drag becomes even more paramount as the single most important determinant of fuel economy in an aircraft.
Commercial off-the-shelf waveguide rotary joints are available. Almost all fall into one of two types, with a central rotating joint employing a coaxial or circular waveguide.
Coaxial rotary joints can be smaller in profile and footprint than those employing a circular waveguide. However, due to the coaxial transmission medium implemented in coaxial rotary joints, they are much lossier and have lower-power handling capabilities relative to circular waveguide rotary joints. Further, when these coaxial rotary joints are used in a rectangular waveguide system, much of the height and profile advantages are negated due to the need to employ multiple coaxial-to-waveguide transitions. Multiple channel alternatives to coaxial rotary joints employ a long cable-wrap comprised of multiple independent coaxial cables, but these are typically limited to 720 degrees or less of rotation and then have to “reset” the cable wrap to avoid permanent damage.
Circular waveguide rotary joints, on the other hand, offer lower loss and improved power-handling capabilities at the expense of a much larger profile and diminished operational frequency bandwidth. The rectangular waveguide sections are attached to the circular waveguide section such that only the TM01 mode is excited inside the circular waveguide. Due to the type of modes excited, this type of construction requires special choking features which significantly increases the height of the assembly and serves to further limit operating frequency bandwidth.